A RC coupling filter in the form of a bandpass filter with two active RC filters is known from German Pat. No. 24 36 966. Although excellent results have been obtained with this double circuit RC coupling filter, it has been considered that further improvement in the properties could be achieved through series connection of several such double circuit RC coupling filters, and in particular an improved response time in comparison to a single double circuit RC coupling filter. Application of series connection of such double circuit RC coupling filters does, of course, create the problem that the coupling factors between the individual double circuit RC coupling filters are not determined by the relationship of the coupling components substantially or objectively present. Hence the problems connected with coupling factors which are eliminated with such series connections reappear to a certain extent, since the detuning coupling factors between the individual series components are as small differences in high frequencies very sensitive to variations in the elements.
In view of the above prior art a multiple circuit (n-circuit, with n.gtoreq.2) RC coupling filter was developed in which optimum selectivity and the shortest possible response times are combined with high stability (insensitivity to the coupling components employed) (U.S. Pat. No. 4,551,686).
In this RC coupling filter, especially an input filter for receivers of centralized ripple control systems, a multiplicity (n, with n.gtoreq.2) of similar four terminal networks was provided each four terminal network having an input and an output and every two adjacent four terminal networks being interconnected by means of two coupling admittances, wherein active RC four terminal networks with at least one amplifier were provided as four terminal networks, the output of the first RC four terminal network being connected by way of a coupling admittance to the input of the second RC four terminal network and the input of the first RC four terminal network being connected by way of another coupling admittance to the output of the second RC four terminal network, wherein, except for the first RC four terminal network and the last RC four terminal network, the input of each RC four terminal network was connected on the one hand by way of a coupling admittance to the output of the preceding RC four terminal network, and on the other, by way of another coupling admittance to the output of the following RC four terminal network, and the output of each RC four terminal network was connected, on the one hand, by way of a coupling admittance to the input of the preceding RC four terminal network, and on the other, by way of another coupling admittance, to the input of the following RC four terminal network and wherein the output of the last RC four terminal network was connected by way of a coupling admittance to the input of the next to last RC four terminal network and the input of the last RC four terminal network was connected by way of another coupling admittance to the output of the next to last RC four terminal network. Unless expressly stated otherwise, when reference is made here and in what follows to an amplifier input, the inverting input of the amplifier is meant. Unless expressly stated otherwise, the noninverting input of the amplifier is in this case connected to the ground.
A RC coupling filter of prior art as indicated in the foregoing makes it possible to use virtually all types of RC four terminal networks. The type of RC four terminal network employed then defines the nature of the entire RC coupling filter. In point of stability, the RC coupling filter is absolutely equivalent to the state of the art (German No. 24 36 966) double circuit RC bandpass filter of the type previously indicated, since all coupling factors represent quotients of coupling components of the same type, so that variations in the coupling components employed have virtually no effect on the coupling factors. It is an aspect of particular significance that all the coupling admittances can be embodied in passive coupling components, specifically, coupling resistances or coupling capacitors.
In this known RC coupling filter each active RC four terminal network preferably has a bypass admittance, two filter admittances, a feedback admittance, and an amplifier. One end of the bypass admittance and of the filter admittances are connected to each other and to the input of the RC four terminal network, and one end of the feedback admittance is connected to the other end of the first filter admittance and to the input of the amplifier. The remaining ends of the second filter admittance and of the feedback admittance, as well as the output of the amplifier, are connected to each other and to the output of the RC four terminal network.
Normally with this prior art RC coupling filter (U.S. Pat. No. 4,551,686) the signal to be filtered is fed into the first RC four terminal network, while output of the filtered signal takes place in the last RC four terminal network. The same alternatives as for the input circuits for the state of the art (German No. 24 36 966) double circuit RC coupling filters apply to the input circuits, which in many cases have an input current converter. In many cases the signal to be filtered is introduced directly into the inverting input of the amplifier of the first RC four terminal network. It may be advantageous, however, if the signal to be filtered is introduced into the noninverting input of the amplifier of the first RC four terminal network. This ensures that the symmetry of the RC four terminal networks is not disrupted by the input. It has already been pointed out that the admittances generally can be embodied in resistances or capacitances. The admittances may be embodied in ohmic resistances or in capacitors in keeping with the filter characteristics which it is desired to obtain. If, for example, for the purpose of design as a bandpass the bypass admittance and the feedback admittance in each RC four terminal network are embodied in ohmic resistances and the filter admittances in capacitors, it is advisable for the coupling admittances to be embodied in capacitors.
In a further aspect of the prior art if the RC coupling filter is designed as a bandpass filter, it may be treated as a truly closed reactance four terminal network, provided that the RC four terminal networks, except the first RC four terminal network and the last RC four terminal network, are fully loss compensated. Loss compensation such as this of the "internal" RC four terminal networks can be accomplished by connecting the non-inverting input of the amplifier of each RC four terminal network, except the first RC four terminal network and last RC four terminal network, by way of a first tuning admittance to the output of the amplifier and by way of a second tuning admittance to the ground. The tuning admittances are in this case preferably embodied in ohmic resistances. The tuning admittances can be adjusted so that the internal losses of the corresponding RC four terminal networks become negligibly small. Hence the "interior" RC four terminal networks are in effect fully loss compensated, so that the number of pole losses can be reduced to two. The RC coupling filter, as a narrow bandpass filter, is thus considerably more stable than any other comparable circuit.
In an alternative aspect of the prior art to the embodiment indicated in the foregoing, with appropriate tuning admittances in the "interior" RC four internal networks, a truly closed reactance four terminal network can also be produced if each of the RC four terminal networks is designed as a resonator with two amplifiers the inverting inputs of which are interconnected. Resonators of this type are already known as such. They have extremely low losses, and thus very slight attenuation at the corresponding frequency. In addition, such resonators are even more insensitive to variations in the elements, and thus are even stabler, since they have a very high open loop voltage gain because of the two amplifiers.
A still further aspect of the prior art is to provide additional circuit potential by designing each RC four terminal network as a multiple stage universal circuit, preferably with a summing stage, two integrating stages, and an inverting stage. A multiple stage universal circuit such as this presents the advantage that the possible inputs and outputs are assigned in advance. Different filter characteristics can be obtained through selection of different inputs or outputs. Such universal circuits are known, for example, as integrated capacitor switch filters in CMOS technology. Such capacitor switch filters have a clock crystal and are thus frequency constant to an extreme degree. Optimum utilization of the advantageous embodiment of the RC coupling filter with the highly stable coupling factor can be achieved precisely with these filters.
In the prior art through appropriate adjustment of the individual admittances in keeping with the design, one may produce all special types of filters such as Bessel filters, Butterworth filters, Tschebyscheff filters, and so forth.
The prior art RC coupling filter as explained above in detail (U.S. Pat. No. 4,551,686) needs a comparably large number of coupling admittances, particularly coupling capacitors. For example with the minimum of two active RC four terminal networks two coupling admittances are necessary. With n RC four terminal networks the number of (n-1).multidot.2 coupling admittances is necessary. This is not only complicated as far as circuitry is concerned but also considerably expensive.